Re-entrant cones for moderator chamber of a neutron imaging system

ABSTRACT

A neutron imaging system that includes a central neutron source configured to produce source neutrons, wherein the central neutron source comprises a beam target, a moderator chamber surrounding at least a portion of the beam target, the moderator chamber housing a moderator, and a re-entrant cone extending into the moderator chamber. The re-entrant cone includes an entrance surface facing the beam target. The entrance surface encloses a cone chamber, isolating the cone chamber from the moderator. Furthermore, the entrance surface is shaped such that source neutrons produced at the beam target impinge the entrance surface with a neutron flux that varies by 10% or less along the entrance surface.

TECHNICAL FIELD

The present disclosure relates generally to the field of neutron imagingsystems (e.g., radiography and tomography systems) and methods thatprovide high-quality, high throughput 2D and 3D fast or thermal neutronimages. Such systems and methods are useful for the commercial-scaleimaging of industrial components.

BACKGROUND

Neutron radiography and tomography are proven techniques for thenondestructive testing and quality control of manufactured components inthe aerospace, energy, automotive, defense, and other sectors. LikeX-rays, when neutrons pass through an object, they provide informationabout the internal structure of that object. Neutrons are able to easilypass through many high-density materials and provide detailedinformation about internal materials, including many low-densitymaterials. This property is important for a number of components thatrequire nondestructive evaluation including jet engine turbine blades,munitions, aircraft and spacecraft components, and composite materials.Historically, commercial neutron radiography used nuclear reactors asthe neutron source. Nuclear reactors are expensive, difficult toregulate, and are becoming increasingly more difficult to access, makingthis powerful inspection technique impractical for many commercialapplications.

Accordingly, a need exists for improved neutron imaging methods andsystems.

SUMMARY

According to a first aspect of the present disclosure, a neutron imagingsystem includes a central neutron source configured to produce sourceneutrons, wherein the central neutron source comprises a beam target, amoderator chamber surrounding at least a portion of the beam target, themoderator chamber housing a moderator, and a re-entrant cone extendinginto the moderator chamber. The re-entrant cone includes an entrancesurface facing the beam target. The entrance surface encloses a conechamber, isolating the cone chamber from the moderator. Furthermore, theentrance surface is shaped such that source neutrons produced at thebeam target impinge the entrance surface with a neutron flux that variesby 10% or less along the entrance surface.

A second aspect includes the neutron imaging system of the first aspect,wherein the entrance surface of the re-entrant cone has a sphericalconcave curvature.

A third aspect includes the neutron imaging system of the first aspector the second aspect, wherein a distance from the entrance surface ofthe re-entrant cone to a center point of the beam target varies by lessthan 10% along the entrance surface.

A fourth aspect includes the neutron imaging system of the first aspector the third aspect, wherein the entrance surface of the re-entrant coneis flat.

A fifth aspect includes the neutron imaging system of any of theprevious aspects, further comprising a neutron collimator extendingoutward from the moderator chamber, wherein the neutron collimator iscoupled to the re-entrant cone such that a neutron pathway extends fromthe entrance surface of the re-entrant cone into the neutron collimator.

A sixth aspect includes the neutron imaging system of the fifth aspect,wherein an inner surface of the neutron collimator is lined with aneutron absorber configured to absorb a portion of the source neutronssuch that the neutron collimator produces a thermal neutron imaging beamline.

A seventh aspect includes the neutron imaging system of any of theprevious aspects, further comprising a neutron imaging detector, whereinthe neutron imaging detector comprises a detector medium and an imagingplane.

An eighth aspect includes the neutron imaging system of the seventhaspect, wherein the detector medium comprises a film, a scintillatingconversion mechanism, or a digital neutron imaging detector.

A ninth aspect includes the neutron imaging system of any of theprevious aspects, wherein the central neutron source comprises aparticle accelerator for generating neutrons from the beam target.

A tenth aspect includes the neutron imaging system of any of theprevious aspects, wherein the re-entrant cone is one of a plurality ofre-entrant cones extending into the moderator chamber in a radial arrayaround the beam target, wherein the entrance surface of each re-entrantcone of the plurality of re-entrant cones faces the beam target.

An eleventh aspect includes the neutron imaging system of any of theprevious aspects, wherein the moderator comprises heavy water and thecone chamber of the re-entrant cone is fluidly isolated from themoderator chamber.

According to a twelfth aspect of the present disclosure, a methodincludes producing source neutrons at a beam target of a central neutronsource of a neutron imaging system, the neutron imaging system furthercomprising a moderator chamber surrounding at least a portion of thebeam target, the moderator chamber housing a moderator and receivingsource neutrons with a re-entrant cone that extends into the moderatorchamber, the re-entrant cone comprising a cone chamber and an entrancesurface facing the beam target, wherein the cone chamber is enclosed bythe entrance surface to isolate the cone chamber from the moderator andthe entrance surface of the re-entrant cone is configured such thatsource neutrons received with the re-entrant cone impinge the entrancesurface with a neutron flux that varies by less than 10% along theentrance surface.

A thirteenth aspect includes the method of the twelfth aspect, whereinthe neutron imaging system further comprises a neutron imaging detectorand a neutron collimator, the neutron imaging detector comprising adetector medium and an imaging plane, the neutron collimator extendsoutward from the moderator chamber, and the neutron collimator iscoupled to the re-entrant cone such that a neutron pathway extends fromthe entrance surface of the re-entrant cone into the neutron collimatorand onto the neutron imaging detector.

A fourteenth aspect includes the method of the thirteenth aspect,further comprising generating a thermal neutron imaging beam linecomprising source neutrons in the neutron collimator and collecting aneutron image of an object positioned at the imaging plane of theneutron imaging detector from portions of the thermal neutron imagingbeam line that passes through the object.

A fifteenth aspect includes the method of the fourteenth aspect, whereinthe object is an airplane part, airplane engine, munition, a productthat utilizes energetic materials, a fuse, rocket, a chemicallyactivated device, a spacecraft part, a wind turbine component, or anaerospace part.

A sixteenth aspect includes the method of the fourteenth aspect or thefifteenth aspect, wherein an inner surface of the neutron collimator islined with a neutron absorber configured to absorb a portion of thesource neutrons such that the neutron collimator produces the thermalneutron imaging beam line.

A seventeenth aspect includes the method of any of the twelfth throughsixteenth aspects, wherein the central neutron source comprises aparticle accelerator for generating neutrons from the beam target.

According to an eighteenth aspect of the present disclosure, a neutronimaging system includes a central neutron source configured to producesource neutrons, wherein the central neutron source comprises a particleaccelerator and a beam target, wherein the beam target is configured toproduce source neutrons upon impingement by a beam accelerated by theparticle accelerator and propagating in a beam direction along a beamplane, a moderator chamber surrounding at least a portion of the beamtarget, the moderator chamber housing a moderator and a re-entrant coneextending into the moderator chamber from a chamber opening of a chamberwall of the moderator chamber, wherein the re-entrant cone comprises anentrance surface facing the beam target, the entrance surface encloses acone chamber, isolating the cone chamber from the moderator, the chamberopening is offset from the beam plane, and the entrance surface isnon-parallel the chamber wall.

A nineteenth aspect includes the neutron imaging system of theeighteenth aspect wherein the entrance surface comprises a first surfaceregion at a location along the entrance surface closest to the beamplane and a second surface region at a location along the entrancesurface farthest from the beam plane and the first surface region isnearer the chamber wall of than the second surface region.

A twentieth aspect includes the neutron imaging system of the eighteenthaspect or the nineteenth aspect wherein the entrance surface is shapedsuch that source neutrons produced at the beam target impinge theentrance surface with a neutron flux having a 50% or greater reductionin variability along the entrance surface compared to a variability ofneutron flux along a reference region located on a reference plane thatintersects a front edge of the entrance surface and is parallel thechamber wall, wherein the reference region is sized to align with thechamber opening.

These and additional features provided by the embodiments describedherein will be more fully understood in view of the following detaileddescription, in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments set forth in the drawings are illustrative and exemplaryin nature and not intended to limit the subject matter defined by theclaims. The following detailed description of the illustrativeembodiments can be understood when read in conjunction with thefollowing drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 schematically depicts a neutron imaging system comprising are-entrant cone, according to one or more embodiments shown anddescribed herein;

FIG. 2 schematically depicts a neutron imaging system comprising anotherre-entrant cone, according to one or more embodiments shown anddescribed herein; and

FIG. 3 schematically depicts a neutron imaging system comprising yetanother re-entrant cone, according to one or more embodiments shown anddescribed herein.

DETAILED DESCRIPTION

Referring generally to the figures, embodiments of the presentdisclosure are directed to neutron imaging systems configured togenerate high resolution, high throughput fast or thermal neutron imagesto provide viable commercial-scale thermal and fast neutron radiography.Neutron radiography and tomography are proven techniques for thenondestructive testing of manufactured components in the aerospace,energy, automotive, defense, and other sectors. Similar to X-rays, whenneutrons pass through an object, they provide information about theinternal structure of that object. However, X-rays interact weakly withlow atomic number elements (e.g., hydrogen) and strongly with highatomic number elements (e.g., many metals). Consequently, their abilityto provide information about low-density materials, particularly when inthe presence of higher density materials, is poor. Neutrons do notsuffer from this limitation. Neutrons pass easily through high densitymetals and provide detailed information about internal materials,including low density materials. Thus, neutrons can be used fornon-destructive evaluation may many components that not conducive toX-rays or other nondestructive evaluation modalities, such as engineturbine blades, munitions, spacecraft components, and compositematerials such as certain aerospace components and wind turbine blades.

Presently, neutron radiography and tomography are underutilized becauseof a lack of accessible, high flux neutron sources with the appropriatespectral characteristics. The neutron imaging systems of the presentdisclosure include accelerator-based neutron sources that can be used inplace of a nuclear reactor or large spallation source. One challenge ofaccelerator-based neutron sources is that they provide several orders ofmagnitude lower source neutrons than a nuclear reactor. Thus, when usingaccelerator-based neutron source, the neutron-detecting medium ispositioned nearer the neutron source than when using a nuclear reactorsource. Indeed, at a nuclear reactor or large spallation source, it istypical that the detection medium can be several meters away from theneutron source, allowing for space in which to place filters to mitigateundesirable types of radiation, mainly stray gammas and fast neutrons,which will partially blur the image during acquisition. It is difficultto place such filters in an accelerator-based neutron source.

Embodiments of the present disclosure are directed to re-entrant conespositioned in a moderator chamber of a neutron imaging system whichoperate to mitigate this undesirable radiation while maximizing thereceipt of desired thermal neutron radiation. The re-entrant cones ofthe present disclosure extend into a moderator chamber (e.g., a heavywater tank) to facilitate the sampling of higher thermal neutronpopulations closer to the target while maintaining a large volume ofheavy water in the moderator chamber. Increasing the volume of heavywater increases the moderation of radiation that does not enter there-entrant cones (i.e., is not used as part of the imaging process). There-entrant cones include an entrance surface shaped to maximize theuniformity of the neutron flux of source neutrons generated by a neutronsource. For example, the re-entrant cones can be shaped to match thenominal constant flux surfaces within the heavy water to increaseneutron uniformity entering the re-entrant cone and thereafter impinginga neutron detector, leading to a higher quality and higher resolutionneutron image. Embodiments of neutron imaging systems will now bedescribed and, whenever possible, the same reference numerals will beused throughout the drawings to refer to the same or like parts.

Referring now to FIGS. 1-3 , a neutron imaging system 100 isschematically depicted. The neutron imaging system 100 includes acentral neutron source 120 comprising a particle accelerator 121 and abeam target 122. The central neutron source 120 is configured to producesource neutrons at the beam target 122. For example, the particleaccelerator 121 accelerates a beam, such as an ion beam, in a beamdirection 10 along a beam plane 15. The source neutrons produced by thecentral neutron source 120 expand radially outward (in neutronpropagation direction 11) from the beam target 122 in ring shapedpatterns, as shown by neutron flux lines 12, 12′, 12″ in FIGS. 1-3 .Using the coordinate system of FIGS. 1-3 , the beam plane 15 is an X-Yplane parallel with the beam direction 10. It is contemplated that theembodiments described herein may utilize several different centralneutron sources 120. For example, the central neutron source 120 maygenerate source neutrons by a deuterium-deuterium (DD) fusion reaction,a deuterium-tritium (DT) fusion reaction, or any other source neutrongenerating reactions using a particle accelerator, such as particleaccelerator 121.

As depicted in each of FIGS. 1-3 , the neutron imaging system 100further comprises a moderator chamber 110 which houses a moderator 105,such as heavy water or graphite, a re-entrant cone 130 that extends intothe moderator chamber 110, a neutron collimator 140 coupled to there-entrant cone 130 at a chamber opening 114 in a chamber wall 112 ofthe moderator chamber 110, and a neutron imaging detector 150. There-entrant cone 130 and the neutron collimator 140 provide a particlepathway for some of the source neutrons exiting the moderator chamber110 to reach a neutron imaging detector 150 and image an object.

The moderator chamber 110 includes one or more chamber walls, such aschamber wall 112, which includes chamber opening 114, the opening fromwhich the re-entrant cone 130 extends into the moderator chamber 110. Inoperation, the moderator 105 attenuates the source neutrons such thatneutron flux reduces as the source neutrons travel away from the beamtarget 122 in a neutron propagation direction 11. For example, theneutron flux of source neutrons at neutron flux line 12 is greater thanthe neutron flux at neutron flux line 12′, which is greater than theneutron flux at neutron flux line 12″. The moderator 105 surrounds atleast part of the beam target 122. The moderator 105 reduces the amountof gamma rays that reach the neutron imaging detector 150 and reducesthe amount of radiation that reaches the one or more chamber walls.

Referring still to FIGS. 1-3 , the re-entrant cone 130 extends into themoderator chamber 110 from a chamber opening 114 in the chamber wall112. The re-entrant cone 130 provides a region that is isolated from themoderator 105 (e.g., fluidly isolated from the heavy water) such thatthe neutron flux of source neutrons traveling within the re-entrant cone130 attenuates at a reduced rate compared to source neutrons propagatingthrough the moderator 105. The re-entrant cone 130 comprises an entrancesurface 132, 132′ that encloses a cone chamber 138, isolating the conechamber 138 from the moderator chamber 110. For example, when themoderator 105 comprises heavy water, the cone chamber 138 is fluidlyisolated from the moderator chamber 110.

The cone chamber 138 may comprise a hollow chamber, a solid chamber(e.g., filled with a moderating material), or a chamber having hollowand filled portions. For example, hollowed portions of the cone chamber138 promote migration of thermal neutrons towards the neutron collimator140 and filled portions continue moderation of the radiation generatedby the central neutron source 120. The hollow portions of the re-entrantcone 130 may house air or other gases and allow for relatively the sameoptical path length for thermal neutrons to enter the neutron collimator140. The filled portions of the re-entrant cone 130 may be composed ofmaterials such as water, high density polyethylene (HDPE), and graphite,for example. Moreover, the one or more re-entrant cones 130 allow for alarger moderator chamber 110 to provide increased radiation shieldingwithout the corresponding reduction in neutron flux at the neutronimaging detector 150. The re-entrant cone 130 may comprise a taperedshape that is cylindrical or rectangular. As depicted in FIGS. 1-3 , thetaper is such cross-sectional shape of re-entrant cone 130 increases asthe re-entrant cone 130 approaches the beam target 122.

Referring still to FIGS. 1-3 , the chamber opening 114 of the chamberwall 112 is offset from the beam plane 15 (e.g., in a −Z direction ofthe coordinate system shown in FIGS. 1-3 ). For example, the chamberopening 114 comprises a centerpoint 115 and the centerpoint 115 isoffset from the beam plane 15 by a distance D_(OFF), offsetting there-entrant cone 130 from the beam plane 15. Without intending to belimited by theory, the thermal neutron population of the source neutronsis significantly more uniform throughout the moderator chamber whencompared to the fast neutron and gamma populations of the sourceneutrons. Offsetting the re-entrant cone 130 from the beam plane 15(e.g., in a −Z direction as shown in FIGS. 1-3 ) offset results in there-entrant cone 130 aiming at the region of nearly highest thermal fluxbut not aiming at the region of highest fast neutron and gamma flux.

Thus, offsetting the chamber opening 114 and the re-entrant cone 130from beam plane 15 reduces the gamma flux (e.g., 2.2 MeV hydrogencapture gammas) and other high energy radiation such as neutrons, thatenters the neutron collimator 140 and reaches the neutron imagingdetector 150, relative to thermal neutrons, improving the resultantimage quality. Moreover, the offset of the beam plane 15 from thechamber opening 114 and the re-entrant cone 130 is large enough toimpede a direct light of sight from the beam target 122 to the chamberopening 114 and thus impede a direct line of sight from the beam target122 to an imaging plane of the neutron imaging detector 150.

Referring now to FIGS. 2 and 3 , the entrance surface 132′ of there-entrant cone 130 faces the beam target 122 is shaped to increase theuniformity of neutron flux entering each re-entrant cones 130, forexample, in comparison to the uniformity of neutron flux entering theentrance surface 132 of FIG. 1 , which is parallel to the chamber wall112 of the moderator chamber 110. Indeed, the entrance surface 132′ maybe shaped to correspond with the neutron flux lines 12, 12′, 12″. Theshape of the entrance surface 132′ may be configured to align with aneutron flux distribution of source neutrons produced at the beam target122. Increasing the uniformity of neutron flux incident across there-entrant cone 130 increases the uniformity of the neutron flux acrossthe field of view of the image captured using the neutron imagingdetector 150, leading to a more consistent exposure across the imageresulting in higher quality neutron images. The entrance surface 132′may be a curved shape, as depicted in FIG. 2 , a flat shape, as depictedin FIG. 3 , or a variable shape. Entrance surfaces 132′ with a curvedshape may have a concave curvature or a convex curvature. As oneexample, the entrance surface 132 comprises a spherical concavecurvature. Moreover, embodiments are contemplated in which the fluxlines follow a variable shape and the entrance surface 132 comprisescorrespondingly variable shape. This variable shape may be determined bya Monte Carlo simulation.

In operation, the re-entrant cone 130 increases the neutron fluxreceived by the neutron imaging detector 150 compared to merely having ahole in the chamber wall 112 (e.g., the chamber opening 114) because theentrance surface 132, 132′ is nearer the beam target 122 than thechamber opening 114 and has a larger surface area than the area of thechamber opening 114. While the small size of the chamber opening 114would increase neutron flux uniformity compared to the entrance surface132 of FIG. 1 , the smaller size of the chamber opening 114 would reducethe total neutron flux that reaches the neutron imaging detector 150. Incontrast, re-entrant cones 130 having the entrance surface 132′ of FIGS.2 and 3 increase the neutron flux incident on the film plane of theneutron imaging detector 150 due to the surface area and positioning ofthe entrance surface 132′, while maintaining sufficient uniformityacross the captured image due to the increased neutron flux uniformityfacilitated by the shape and orientation of the entrance surface 132′.

As shown in FIGS. 2 and 3 , the entrance surface 132′ comprises a firstsurface region 134 at a location along the entrance surface 132′ closestto the beam plane 15 and a second surface region 136 at a location alongthe entrance surface 132′ farthest from the beam plane 15. The firstsurface region 134 is nearer the chamber wall 112 of than the secondsurface region 136. This orients the entrance surface 132′ toward thebeam target 122. Referring still to FIGS. 2 and 3 , entrance surface132′ is non-parallel the chamber wall 112 (which is the wall from whichthe re-entrant cone 130 extends into the moderator chamber 110).

FIGS. 2 and 3 further depict a reference plane 131 that intersects afront edge of the entrance surface 132′ (the front edge correspondingwith the second surface region 136) and is parallel the chamber wall112. The entrance surface 132′ is shaped such that source neutronsproduced at the beam target impinge the entrance surface 132′ with aneutron flux having a 25% or greater reduction in variability along theentrance surface 132′ compared to a variability of neutron flux along areference region located on the reference plane 131 where the referenceregion is the portion of the 132′ is sized to align with the chamberopening 114. For example, this reduction in variability may be 25% orgreater, 30% or greater, 35% or greater, 40% or greater, 45% or greater,50% or greater, 55% or greater, 60% or greater, 70% or greater, 80% orgreater, 90% or greater, 95% or greater, 99% or greater, or a value in arange having any two of these values as endpoints.

In some embodiments, a distance from the entrance surface 132′ of there-entrant cone 130 to a beam target centerpoint 125 of the beam target122 varies by 20% or less along the entrance surface 132, for example,by 18% or less, 15% or less, 12% or less, 10% or less, 9% or less, 8% orless, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% orless, 1.5% or less, 1% or less, 0.5% or less, 0.1% or less, or the like.In some embodiments, the entrance surface 132′ is shaped such thatsource neutrons produced at the beam target 122 impinge the entrancesurface 132 with a neutron flux that varies by 20% or less along theentrance surface 132, for example, by 18% or less, 15% or less, 12% orless, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less, 5% orless, 4% or less, 3% or less, 2% or less, 1.5% or less, 1% or less, 0.5%or less, 0.1% or less, or a value in a range having any two of thesevalues as endpoints.

Referring again to FIGS. 1-3 , the re-entrant cone 130 provides apathway for source neutrons to exit the moderator chamber 110 and enterthe neutron collimator 140. The neutron collimator 140 provides aparticle pathway for some of the source neutrons exiting the moderatorchamber 110 to reach the neutron imaging detector 150. The neutroncollimator 140 extends outward from the moderator chamber 110. Theneutron collimator 140 is coupled to the re-entrant cone 130 and thechamber opening 114. Thus, a neutron pathway extends from the entrancesurface 132, 132′ of the re-entrant cone 130 into the neutron collimator140. An inner surface 145 of the neutron collimator 140 is lined with aneutron absorber configured to absorb a portion of the source neutronsand produce a thermal neutron imaging beam line 20, for example, fromthermal neutrons of the source neutrons. This thermal neutron imagingbeam line 20 impinges the neutron imaging detector 150 to image anobject. In some embodiments, the neutron absorber positioned along theinner surface 145 of the one or more neutron collimators 140 is selectedfrom the group consisting of: cadmium, boron and boron-containingcompounds, lithium and lithium-containing compounds, gadolinium, andcomposites containing any of these materials.

Referring still to FIGS. 1-3 , the neutron imaging detector 150comprises a detector medium and an imaging plane. The detector mediummay comprise a film (e.g., a radiographic film), a scintillatingconversion mechanism, a storage phosphor, a direct conversion screen, anamorphous silicon flat panels, a microchannel plate, a digital detectorarray, and/or an indirect conversion screen. In some embodiments, theneutron imaging detector 150 is a non-planar neutron detector thatconforms to the contour of the object to be imaged to minimize theblurring effect from a thermal neutron imaging beam line 20 that isnon-parallel with the object. In such instances, the non-planar detectormay comprise film or digital media, such as scintillating materialcoupled to light transmitting, converting, multiplying, and/or detectorelements such as fiber optic guides and photomultiplier tubes. While notdepicted, in some embodiments the neutron imaging system 100 may furthercomprise neutron focusing and/or reflecting elements which areconfigured to increase neutron flux at the imaging plane of the neutronimaging detector 150 to increase image resolution.

An imaging operation using the neutron imaging system 100 includesproducing source neutrons at the beam target 122, receiving sourceneutrons with the re-entrant cone 130 such that source neutrons enterthe neutron collimator 140, generating a thermal neutron imaging beamline 20 in the neutron collimator 140, and collecting a neutron image ofan object positioned at the imaging plane of the neutron imagingdetector 150 from portions of the thermal neutron imaging beam line 20that pass through the object, thereby generating a neutron image. Insome embodiments, the object is an airplane part (e.g., wings), airplaneengine, munition, a product that utilizes energetic materials, a fuse,rocket, a chemically activated device, a spacecraft part, a wind turbinecomponent, (e.g., a composite part), or an aerospace part.

The neutron imaging techniques described herein may be combined withother nondestructive evaluation techniques, including X-ray radiographyand tomography, to create fusion image data sets that provide moreinformation than a standalone neutron image or x-ray image. Othernondestructive evaluation techniques that provide 2D and 3D informationabout a component that may be fused with the neutron image includeultrasound, magnetic resonance, magnetic penetrant, thermography, x-rayfluorescence, and small angle neutron scattering, amongst others. Insuch cases, image registration software may be used to correlate datafrom two or more nondestructive evaluation techniques to create a fusionimage data set.

Referring again to FIGS. 1-3 , while a single re-entrant cone 130 andsingle neutron collimator 140 are depicted for ease of understanding theconcepts described herein, it should be understood that embodiments arecontemplated comprising a plurality of re-entrant cones extending intothe moderator chamber, for example, in a radial array around the beamtarget 122. In such embodiments, each of the plurality of re-entrantcones may comprise the entrance surface 132′ described herein orientedat the beam target and each may be coupled to a corresponding neutroncollimator. Moreover, the plurality of re-entrant cones may extend intothe moderator chamber 110 in a radial array along a common plane, forexample, a common X-Y plane passing through the centerpoint 115 of thechamber opening 114, offset from the beam plane 15, for example, offsetalong the Z-axis as depicted in FIGS. 1-3 .

While particular embodiments have been illustrated and described herein,it should be understood that various other changes and modifications maybe made without departing from the spirit and scope of the claimedsubject matter. Moreover, although various aspects of the claimedsubject matter have been described herein, such aspects need not beutilized in combination. It is therefore intended that the appendedclaims cover all such changes and modifications that are within thescope of the claimed subject matter.

As utilized herein, the terms “approximately,” “about,” “substantially”,and similar terms are intended to have a broad meaning in harmony withthe common and accepted usage by those of ordinary skill in the art towhich the subject matter of this disclosure pertains. It should beunderstood by those of skill in the art who review this disclosure thatthese terms are intended to allow a description of certain featuresdescribed and claimed without restricting the scope of these features tothe precise numerical values or idealized geometric forms provided.Accordingly, these terms should be interpreted as indicating thatinsubstantial or inconsequential modifications or alterations of thesubject matter described and claimed are considered to be within thescope of the disclosure as recited in the appended claims.

The term “coupled” and variations thereof, as used herein, means thejoining of two members directly or indirectly to one another. Suchjoining may be stationary (e.g., permanent or fixed) or moveable (e.g.,removable or releasable). Such joining may be achieved with the twomembers coupled directly to each other, with the two members coupled toeach other using a separate intervening member and any additionalintermediate members coupled with one another, or with the two memberscoupled to each other using an intervening member that is integrallyformed as a single unitary body with one of the two members. If“coupled” or variations thereof are modified by an additional term(e.g., directly coupled), the generic definition of “coupled” providedabove is modified by the plain language meaning of the additional term(e.g., “directly coupled” means the joining of two members without anyseparate intervening member), resulting in a narrower definition thanthe generic definition of “coupled” provided above. Such coupling may bemechanical, electrical, or fluidic.

References herein to the positions of elements (e.g., “top,” “bottom,”“above,” “below”) are merely used to describe the orientation of variouselements in the FIGURES. It should be noted that the orientation ofvarious elements may differ according to other exemplary embodiments,and that such variations are intended to be encompassed by the presentdisclosure.

Although the figures and description may illustrate a specific order ofmethod steps, the order of such steps may differ from what is depictedand described, unless specified differently above. Also, two or moresteps may be performed concurrently or with partial concurrence, unlessspecified differently above. Such variation may depend, for example, onthe software and hardware systems chosen and on designer choice. Allsuch variations are within the scope of the disclosure. Likewise,software implementations of the described methods could be accomplishedwith standard programming techniques with rule-based logic and otherlogic to accomplish the various connection steps, processing steps,comparison steps, and decision steps.

What is claimed is:
 1. A neutron imaging system comprising: a centralneutron source configured to produce source neutrons, wherein thecentral neutron source comprises a beam target; a moderator chambersurrounding at least a portion of the beam target, the moderator chamberhousing a moderator; and a re-entrant cone extending into the moderatorchamber, wherein: the re-entrant cone comprises an entrance surfacefacing the beam target; the entrance surface encloses a cone chamber,isolating the cone chamber from the moderator; and the entrance surfaceis shaped such that source neutrons produced at the beam target impingethe entrance surface with a neutron flux that varies by 10% or lessalong the entrance surface.
 2. The neutron imaging system of claim 1,wherein the entrance surface of the re-entrant cone has a sphericalconcave curvature.
 3. The neutron imaging system of claim 1, wherein adistance from the entrance surface of the re-entrant cone to a centerpoint of the beam target varies by less than 10% along the entrancesurface.
 4. The neutron imaging system of claim 1, wherein the entrancesurface of the re-entrant cone is flat.
 5. The neutron imaging system ofclaim 1, further comprising a neutron collimator extending outward fromthe moderator chamber, wherein the neutron collimator is coupled to there-entrant cone such that a neutron pathway extends from the entrancesurface of the re-entrant cone into the neutron collimator.
 6. Theneutron imaging system of claim 5, wherein an inner surface of theneutron collimator is lined with a neutron absorber configured to absorba portion of the source neutrons such that the neutron collimatorproduces a thermal neutron imaging beam line.
 7. The neutron imagingsystem of claim 1, further comprising a neutron imaging detector,wherein the neutron imaging detector comprises a detector medium and animaging plane.
 8. The neutron imaging system of claim 7, wherein thedetector medium comprises a film, a scintillating conversion mechanism,or a digital neutron imaging detector.
 9. The neutron imaging system ofclaim 1, wherein the central neutron source comprises a particleaccelerator for generating neutrons from the beam target.
 10. Theneutron imaging system of claim 1, wherein the re-entrant cone is one ofa plurality of re-entrant cones extending into the moderator chamber ina radial array around the beam target, wherein the entrance surface ofeach re-entrant cone of the plurality of re-entrant cones faces the beamtarget.
 11. The neutron imaging system of claim 1, wherein the moderatorcomprises heavy water and the cone chamber of the re-entrant cone isfluidly isolated from the moderator chamber.
 12. A method comprisingproducing source neutrons at a beam target of a central neutron sourceof a neutron imaging system, the neutron imaging system furthercomprising a moderator chamber surrounding at least a portion of thebeam target, the moderator chamber housing a moderator; and receivingsource neutrons with a re-entrant cone that extends into the moderatorchamber, the re-entrant cone comprising a cone chamber and an entrancesurface facing the beam target, wherein: the cone chamber is enclosed bythe entrance surface to isolate the cone chamber from the moderator; andthe entrance surface of the re-entrant cone is configured such thatsource neutrons received with the re-entrant cone impinge the entrancesurface with a neutron flux that varies by less than 10% along theentrance surface.
 13. The method of claim 12, wherein: the neutronimaging system further comprises a neutron imaging detector and aneutron collimator; the neutron imaging detector comprising a detectormedium and an imaging plane; the neutron collimator extends outward fromthe moderator chamber; and the neutron collimator is coupled to there-entrant cone such that a neutron pathway extends from the entrancesurface of the re-entrant cone into the neutron collimator and onto theneutron imaging detector.
 14. The method of claim 13, furthercomprising: generating a thermal neutron imaging beam line comprisingsource neutrons in the neutron collimator; and collecting a neutronimage of an object positioned at the imaging plane of the neutronimaging detector from portions of the thermal neutron imaging beam linethat passes through the object.
 15. The method of claim 14, wherein theobject is an airplane part, airplane engine, munition, a product thatutilizes energetic materials, a fuse, rocket, a chemically activateddevice, a spacecraft part, a wind turbine component, or an aerospacepart.
 16. The method of claim 14, wherein an inner surface of theneutron collimator is lined with a neutron absorber configured to absorba portion of the source neutrons such that the neutron collimatorproduces the thermal neutron imaging beam line.
 17. The method of claim12, wherein the central neutron source comprises a particle acceleratorfor generating neutrons from the beam target.
 18. A neutron imagingsystem comprising: a central neutron source configured to produce sourceneutrons, wherein the central neutron source comprises a particleaccelerator and a beam target, wherein the beam target is configured toproduce source neutrons upon impingement by a beam accelerated by theparticle accelerator and propagating in a beam direction along a beamplane; a moderator chamber surrounding at least a portion of the beamtarget, the moderator chamber housing a moderator; and a re-entrant coneextending into the moderator chamber from a chamber opening of a chamberwall of the moderator chamber, wherein: the re-entrant cone comprises anentrance surface facing the beam target; the entrance surface encloses acone chamber, isolating the cone chamber from the moderator; the chamberopening is offset from the beam plane; and the entrance surface isnon-parallel the chamber wall.
 19. The neutron imaging system of claim18, wherein: the entrance surface comprises a first surface region at alocation along the entrance surface closest to the beam plane and asecond surface region at a location along the entrance surface farthestfrom the beam plane; and the first surface region is nearer the chamberwall of than the second surface region.
 20. The neutron imaging systemof claim 18, wherein the entrance surface is shaped such that sourceneutrons produced at the beam target impinge the entrance surface with aneutron flux having a 50% or greater reduction in variability along theentrance surface compared to a variability of neutron flux along areference region located on a reference plane that intersects a frontedge of the entrance surface and is parallel the chamber wall, whereinthe reference region is sized to align with the chamber opening.